ever since i bought my car, when i drive it hard, (100mph+ or fast in the city) i smell something similar to the smell of a firework, gun powder, sulphury. I always sort of assumed it was clutch, but lately I think, with my suffering gas mileage, interesting engine codes and pungeant odor...i wonder if it isn't symptons of anything more along the lines of carbon buildup.

what should i be checking/changing/cleaning?